PROJECT ABSTRACT Ovarian cancer patients continue to have a high mortality rate due to late stage diagnoses and frequent relapse. Ovarian cancer is not exclusively confined to the peritoneal cavity. Disseminated and circulating tumor cells are often detected and are associated with worse survival outcomes. Therefore, focused efforts are needed to identify factors that permit ovarian cancer cells to escape the peritoneum and identify sites that offer safe harbors where the tumor cells can evade chemotherapy and be reactivated to cause relapse. Low numbers of disseminated ovarian tumor cells have been detected in the bone marrow, however, mechanisms for bone marrow homing and engraftment are not known. There is strong evidence that aberrant Rac1 GTPase signaling contributes to tumor metastasis, invasion and survival, based on roles as a regulator of cell-cell adhesion, actin reorganization and cell motility. Furthermore, Rac1 is crucial for engraftment and quiescence of hematopoietic cells in the bone marrow niche. The Wandinger-Ness group previously reported that Rac1 is overexpressed and the constitutively active Rac1b splice variant of Rac1, is elevated in high grade serous ovarian tumors. Conversely, inhibition of Rac1 through perioperative use of a Rac1/Cdc42 dual inhibitor, was associated with improved patient survival. Taken together, the data suggest that Rac1 GTPase may be an important driver in ovarian cancer dissemination and enable engraftment in a protected niche such as the bone marrow from which relapse may originate. The present proposal will test the hypothesis that Rac1 overexpression or hyperactivation promotes ovarian cancer metastasis, and leads to tumor cell dissemination into the bone marrow and establishment of a quiescent, cell population. Through a combination of in vitro cell based assays and xenograft animal model studies, the impact of Rac1 overexpression and inhibition on invasion, metastasis and bone marrow homing and quiescence will be tested. The experimental data will be used to parameterize a computational model designed to simulate ovarian cancer cell homing to the bone and identify the most critical nodes in the process that might serve as targets. Collectively, these studies will establish Rac1 as driver of ovarian cancer cell dissemination and validate Rac1 as a high value therapeutic target with potential impact in reducing ovarian cancer disease relapse.